The commonwealth of cells

by H. G. F. Spurrell

This diagram shows how certain cells of the outer layer are budded off and transferred to a safe place within the body. In this position the cells are further developed, throwing out one long fibre, which goes to some distant organ of the body, and short fibres, which, though they do not join those of other cells and become continuous, closely interlace and put them into communication. They are also separated from one another by connective tissue, which supports them, holding them suspended with only their fibres approaching one another ([Diagram 43]). [Diagram 44] shows how the bone which replaces the supporting rod (see [Diagram 6]) throws an arch round the feltwork of connective tissue in which the nerve cells are suspended, giving them still further protection.

It will be noticed in the figures of [Diagram 42], which is fuller than [Diagram 5], that there are three of these buds—one central and two lateral. The central one becomes a tube running the whole length of the animal, while the lateral buds form solid clusters or ganglia, arranged in pairs at intervals beside it ([see Diagram 45]). Fibres from these ganglia go to the skin, and bring to the nerve cells information from the outside world, which they duly pass on to the cells of the central column. The cells of the central column, when set in motion by the ganglion cells, send out impulses to the muscles, whose contraction is necessary to perform the movement which circumstances indicate. A movement brought about in this way is called reflex.

Diagram 45.—Central Nerve Tube and Ganglia.

The reflex movements are, however, not quite the simplest. For instance, the food is moved along the alimentary canal by the contraction of two sets of muscle fibres—an outer longitudinal coat and an inner circular one. Between these two coats are some nerve cells, which are thrown into activity by the presence of food and the iron compounds of the bile secreted by the liver in the tube. These sympathetic cells do not send their impulses to any centre for examination, but at once stimulate the muscle fibres between which they lie, thereby producing the peristaltic movements we have already described. Yet it should be remembered that, though these cells act independently of the central nervous system, they are under its control, and can, if need be, have their action modified for the benefit of the body as a whole.

For convenience’ sake, we had better here specify the chief kinds of nervous action. First there is what we may call the immediate nerve action, such as that we have just been describing; secondly there is reflex action, the centres for which are in the spinal cord and the base of the brain; and thirdly there is voluntary movement, which arises out of the interaction of centres in the hemisphere of the brain, where the most complex machinery of all is kept.

II.

Of the first kind we need say no more. The instance of peristaltic movement illustrates it sufficiently; so we can at once begin a more careful examination of reflex action.

The simplest instance of reflex action may be taken from the schoolroom. If a boy suddenly sticks a pin into an unsuspecting schoolfellow, the latter invariably starts, and frequently lets fall an exclamation also. In this case the presence of an injurious agency is reported to the nearest motor centre, which is in the spinal cord, and this automatically convulses the body, jerking the limb out of danger.

This is reflex movement; the nerve fibre, which conveys an intimation of the injurious influence, is a prolongation, or really two prolongations, of a spinal ganglion cell. ([See Diagram 46.]) The near end of this fibre, which enters the cord, has several branches. Some run a little way up the cord, and some a little way down, so as to communicate with several motor cells; but one branch runs right up the cord, and sends the message on to the brain. Our outraged schoolboy starts a fraction of a second before he is conscious of the pain of being pricked, and this first response is involuntary and unvarying; the sensation, however, is reported to his brain, and the workings of that wonderful organ are less easy to predict. It leads to his taking stock of the aggressor, on the strength of which he decides whether it is safe to attempt a reprisal, and, if so, in what form it will be most effective and least likely to attract the master’s attention. This knotty point settled, the motor cells of the brain send down messages to the motor cells of different parts of the spinal cord, and these in turn set the necessary muscles in motion for delivering a surreptitious kick or aiming a splash of ink, as the case may be. This is voluntary movement.